In graphic arts technology, a number of well-established printing processes utilize image carriers with three-dimensional (3D) representation of data, the most popular of them being flexographic printing, which uses flexible relief plates or sleeves. The relief is composed of the raised features on the plate such as the features labeled 204, 208, and 212 in FIG. 2. It is the relief that accepts and transfers ink to the substrate. In a traditional flexographic prepress process with chemical etching there is no possibility of fine control of relief properties other than depth of relief 216.
Flexographic printing uses a flexible relief plate to print on a wide variety of substrates including paper, cardboard, plastic, and metal films. A simplified diagram of a flexographic printing press is shown in FIG. 1. Ink 10 in a fountain pan is taken up by a rubber roller 12 and transferred to the surface of the Anilox roller 14. The surface of the Anilox roller is composed of an array of indented cells that allow careful metering of the ink volume. A doctor blade 16 removes any excess ink from the roller before the ink is transferred to the printing plate cylinder 18. Mounted on the plate cylinder is a flexographic printing plate 20. The final step transfers the ink from the plate to the substrate 22 with the impression cylinder 24 supplying support for the substrate.
The process used to produce an image on flexible relief plate usually comprises the following steps:
Exposing the back of the plate to UV light;
Exposing an intermediate film to the desired image;
Laminating the film to the top of the plate;
Exposing the plate though the film using UV light;
Removing the film;
Using a solvent to wash away the unexposed plate material;
Applying additional exposure to harden the plate; and
Drying the plate to remove as much of the solvent as possible.
The back exposure is used to establish the floor of the plate. The intensity of the exposure decreases as the illumination penetrates the plate because of absorbers added to the plate material. Once the intensity drops below a threshold value, there is insufficient cross linking in the polymer comprising the plate and the remaining under-exposed polymer can be washed away. This is usually the top 0.5 mm of the plate. To form the relief, the front of the plate is exposed, through an image layer with enough intensity that sufficient cross linking occurs all the way down to the plate floor.
For every opening in the image layer, a cone of UV light with an angle of about 40 degrees from a normal to the plane propagates through the plate forming cone shaped relief dots. A cross-section of a plate 200 is shown in FIG. 2. The following features are depicted in the cross-section 200: a solid area 204; an isolated dot 208; and an array 212 of closely spaced dots created by a halftone screen. The height of the plate relief is shown by numeral 216 and plate floor by numeral 220.
Ink uniformity and density can be improved if a surface pattern or texture is applied to the flat tops of the relief as shown in the FIG. 3. The stretched checkerboard pattern 304 is composed of 5×10 micron rectangles and works well for process inks printed on a paper substrate.
Such a fine pattern has an additional advantage in that it allows the edges of printing features to be well defined. The pattern does have its limits. When printing on plastic substrates, voids can appear in large features due to air entrapment. The pattern also performs poorly if large volumes of ink need to be transferred to the substrate. To eliminate these problems, a coarser pattern is required. However, a coarser pattern will compromise edge definition.
In flexographic printing, large solid areas of relief can suffer from a number of artifacts. The ink deposits unevenly, resulting in a reduction in color density and in a mottled appearance to the solid. Ink can be squeezed off the relief near edges resulting in low ink density just inside the edge and high density just outside the edge. Air bubbles trapped between the plate and substrate can cause voids to appear at the trailing edge of large features. Prior art exists to mitigate some these problems as described below.
Early flexography printing relied on a flat, smooth surface for the relief. FIG. 4 shows a section of halftone with a smooth relief. In large solid regions of image, the ink deposition was uneven resulting in a reduction in measured ink density. With high impression force, ink often squeezed out at relief edges reducing ink density just inside the edge with a ring of high density ink just outside the edge.
One method of improving the performance of the plate is to apply a very fine pattern 504, shown in FIG. 5, to the surface of the relief. This creates a texture that is smaller than the resolution of the flexographic printing method. The stretched checkerboard of FIG. 5 is one such example.
The dimensions of the pixels shown in FIG. 5 are 5.3 by 10.6 microns. The pixels that form on the plate are slightly smaller, creating small gaps between pixels at the corners. The edges of the pixels fall off at an angle of about 40 degrees to a valley floor, 3 to 4 microns below the relief surface. This shape allows ink to settle in the valleys and to migrate between valleys at the pixel corners. The result is a more even deposition of ink on the substrate.
Some imaging devices used to make flexographic plates do not have sufficient resolution to image very fine textures. For these devices, another method using coarser textures was developed. The problem with coarser textures is that the dot edge can be compromised. To avoid this, patterning is suppressed a set distance from the dot edge (the keep-away). FIG. 10 shows one implementation of this method. A coarser pattern 1004 is used in the interior of the relief and a 2 pixel keep-away 1008 is implemented at the edges. This keep-away 1008 conserves the shape of the dot.
The pattern shown in FIG. 10 is a regularly spaced pattern but this is not a requirement of the method. Stochastic screening methods have been used to randomly locate voids in the relief areas without violating the keep-away rule. An example of the stochastic method can be seen in the lower left corner of FIG. 3 identified as ‘Traditional Plate Cell Patterning’ 308.
To overcome the weakness of these methods, this invention combines a fine pattern at the edges of printing features with a coarser pattern in the interior of the features.